For how many days did Israel mourn Moses in the plains of Moab?

Answer: Thirty

The Israelites grieved for Moses in the plains of Moab thirty days, until the time of weeping and mourning ended.

“The Israelites grieved for Moses in the plains of Moab thirty days, until the time of weeping and mourning for Moses came to an end.”— Deuteronomy 34:8 (BSB)